Unusual pastry, split evenly by calorie (8)
I believe the answer is:
atypical
'unusual' is the definition.
(synonyms)
'pastry split evenly by calorie' is the wordplay.
'evenly' indicates alternate letters (the letters in even-numbered positions).
'by' says to put letters next to each other.
'calorie' becomes 'cal'.
'pastry'+'split'='pastrysplit'
The alternating letters of 'pastrysplit' are 'atypi'.
'atypi'+'cal'='ATYPICAL'
(Other definitions for atypical that I've seen before include "Not as expected" , "Not conforming to a norm or average" , "'Abnormal, aberrant (8)'" , "Abnormal, aberrent." , "Odd" .)